Exported HAR is not following HAR 1.2 specification

Generating HAR using sitespeed.io (that uses this repo) results in a HAR file not following [HAR 1.2 specification](http://www.softwareishard.com/blog/har-12-spec/).

Generating HAR from visiting https://webperf.se results in the following error:
```
Error: log.entries[0].response.content.compression has wrong value type, actual value type:
```
According to spec (IF I read it correctly) this should be of type `number` and not `string`.

```
Error: log.entries[0].response.content.text has wrong value, actual value: , content-type: text/html;charset=utf-8
```
According to spec (IF I read it correctly) this should (if present) include the actual content of the resource, either in plain text or encoded.
Sadly today, this value is always an empty string.
This part is related to #22